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Tuesday, July 25, 2017

Major Dry-Day Records May Be Broken

As July approaches its end, it is becoming increasingly likely that Seattle and other northwest locations may break some major records regarding lack of rain:

1. In Seattle, the first July without a trace of rain and the driest July on record.
2. The longest run of days without measurable precipitation in Seattle. (measurable rain means at least .01 inch of rain). The current record is 51 days and today we have gotten through 38 dry days in a row.

The last month has been quite dry....but it is important to note that this IS the driest time of the year in the Northwest and we are one of driest places in the country during midsummer. To illustrate the situation, here is the percent of average precipitation for the past month (6/24-7/23). Much of the western U.S., including western Washington, Oregon, and California,t has received less than 2% of the normal amounts.

Now this sounds very scary, but it really isn't as extreme and impactful as you might think, considering we are now in the West Coast dry season, which enjoys a Mediterranean Climate (wet winter, dry summer). To illustrate, here is the difference of the precipitation of the past month from normal (climatology). Most of California and Oregon are less than an inch drier than normal. Western Washington is the most anomalous, with 1-2 inches below normal.

The origin of our dry period is fairly clear: persistent high pressure over the West Coast, something illustrated by this graph, which shows the difference from normal of recent upper atmosphere (500 hPa) heights (green/yellow signifies above average).

We are not only dry, but are entering the driest time of the year, as illustrated from this plot of the probability of .01 inch of precipitation in a day.

So what is the forecast?

Let's start with the best extended predictions: the 24h precipitation forecasts of the European Center ensemble of 51 forecasts out to 9 August. The top panel shows the individual precipitation forecasts, with most showing nothing through the end of the month, with only about half showing very light rain during early August. The bottom panel shows the ensemble mean (or average), which is typically a very good forecast.

The bottom line: the July record looks probable and we have a good chance and beating the 51 day record.

To get another view of forecasts, here is the output of the NAEFS ensemble--a combination of the U.S. and Canadian ensemble forecasts--through August 8. The second panel shows precipitation forecasts (in mm). Dry though July and a small chance of very light precipitation after August 4th.

More support of us having a real shot at the 51 day consecutive dry-day record.

One interesting issue: the dry weather has been associated with very little lightning, resulting in only a modest number of wildfires so far.

Cliff:Yes,I understand.But the 1891 to 1948 time period is just as legitimate.By the way,it would be nice to get an explanation from the NCDC or WRCC why the 1945 through 1947 Sea-Tac data has been excluded from the records file.This only started a few years back,and I don't understand the reasoning behind it.

I have few complaints about the weather we've enjoyed in 2017. Yes, we had a protracted wet spring, but that precipitation saturated our soils and dusted our glaciers just in time for the tap to run dry.

We have had little in the way of heat or humidity so far this summer. This warm and slightly humid patch we've experienced the past few days has been the exception rather than the rule. Most of our nights have been gloriously crisp. I've even noticed fewer mosquitoes.

Whatever all you meteorologist elves have been doing to make this happen, keep it up.

Shouldn't the plot of the probability of .01 inch of precipitation in a day be a smooth graph instead of having all those ups and downs? I agree that if the graph is strictly based upon the record over a number of years, then it would look jagged, as it is drawn. However, it seems that a more theoretically correct representation of the probability would be a graph of a function that is gradually decreasing in parts of its domain and gradually increasing in others.